docs(nr-k8s-otel-collector): rewrite collector.md with complete BYO collector guide

[dpacheconr]

Summary

• Rewrites collector.md as a comprehensive guide for bringing your own OpenTelemetry Collector to send data to New Relic
• Provides complete, copy-pasteable DaemonSet and Deployment configurations with all receivers, processors, exporters, and pipelines
• Adds platform-specific sections for OpenShift and GKE Autopilot
• Adds a guide for using the OpenTelemetry Kube Stack Helm chart, including the ${file:/path} confmap provider workaround for per-node CPU/memory percentage calculations
• Documents RBAC requirements, collector self-metrics, and deployment steps with init container setup

Test plan

• Validated kube-stack deployment locally in Minikube
• Confirmed node.cpu.usage.percentage and node.memory.usage.percentage metrics arrive in New Relic
• Verified all internal links and example file references resolve correctly

[dpacheconr]

Rebased onto master.